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We have been developing a thin-foil-nested X-ray telescope (XRT) for the Japanese X-ray astronomy satellites
since ASCA launched in 1993. The thin-foil-nested XRT is advantageous in realizing high throughput with a
light weight, whilst its angular resolution is poorer than other mirrors using, for example, a polished thick glass
substrate onboard Chandra. We have investigated causes of image degradation of our XRT, and have identified
them as figure error and positioning error of the reflectors. Since the latter dominates the entire error budget, we
first attempted to improve the positioning error, and adopted the following two methods. First, we substituted
an alignment plate for the alignment bar. The alignment plate incorporates the independently movable four
alignment bars adopted for ASCA and Suzaku into a single plate. In practice, we allocated a pair of the plate in
each slot, and after inserting all the reflectors, we shifted one of the plates radially to tightly hold the reflectors.
Second, we have manufactured a partially replicated foil in which reflecting material (Au) is not applied to the
axial edges of the foils in order to control the foil position by the edges of the reflectors whose thickness variation
is within 2 μm. After these improvements on the positioning error, we moved onto the figure error issue. With
X-ray and laser profilometer measurements, we found that the figure error increased in a area closer to the
azimuthal edges of the reflector. After considerable struggle for improvement, we finally decided to produce a
long reflector and to cut both azimuthal edges. Thanks to these new devices, we have reduced the positioning
error from 1.5 arcmin to 0.66. Furthermore, we have successfully produced 40-pairs of reflectors whose figure
error is less than 0.8 arcmin. Incorporating these reflectors into a mirror housing, we have measured the XRT
performance in the 30 m beamline facility at ISAS/JAXA, and confirmed to achieve an angular resolution of 1.08
arcmin in half-power diameter. The effective area is measured to be 14.0 cm2, which is ~90% of the designed
value. Note that this number is significantly enhanced from ~80% in the Suzaku XRT, which is a by-product of
the improvement of the angular resolution.
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